The present disclosure relates, in various exemplary embodiments, to organic light emitting devices (OLEDs). In particular, the present disclosure relates to stacked OLED configurations.
Organic light emitting devices (OLEDs) represent a promising technology for display applications. A typical organic light emitting device includes a first electrode; a luminescent region comprising one or more electroluminescent organic material(s); and a second electrode; wherein one of the first electrode and the second electrode functions as a hole injecting anode, and the other electrode  functions as an electron injecting cathode; and wherein one of the first electrode and the second electrode is a front electrode, and the other electrode is a back electrode. The front electrode is transparent (or at least partially transparent) while the back electrode is usually highly reflective to light. When a voltage is applied across the first and second electrodes, light is emitted from the luminescent region and through the transparent front electrode.
It is sometimes desirable to laminate two or more individual OLEDs in a stacked configuration to form a stacked OLED. Stacked OLED configurations include intermediate electrodes disposed between adjacent individual OLEDs. That is, successive OLEDs share an intermediate electrode, and a top electrode of one individual OLED in the stack also functions as a bottom electrode of another OLED in the stack. The intermediate electrodes are generally transparent. Further, the intermediate electrodes are often required to act as electron injecting contacts on one side and as hole injecting contacts on the other side.
Stacked OLEDs may emit different colors such that a true color pixel is formed from which any color may be emitted. For example, Burrows et al. disclose in Appl. Phys. Lett. 69, 2959 (1996) individual OLEDs with red, green, or blue emissions stacked to form color-tunable vertically integrated pixels.
Stacked, monochromatic OLEDs are also possible such as demonstrated by Matsumoto et al. (SID 03 Digest, 979 (2003)). Stacked, monochromatic OLEDs potentially provide an OLED configuration having high electroluminescence efficiency.
While the above described stacked OLEDs demonstrate configurations allowing for variable emission colors and monochromatic OLEDs with high electroluminescence efficiency (e.g., greater than 10 cd/A), both suffered from a limited operational stability. Limited operational stability is a known problem for OLEDs in general.
Thus, there is still a need for stacked OLED configurations that exhibit the advantages of existing stacked OLED configurations. There is also a need for stacked OLED configurations that are capable of demonstrating increased operational stability.